When looking at any living creature, you should ask yourself two things: Does it have a jaw? Does it have a spinal column? If the answer to these questions is yes, then you share a common ancestor with it — a very distant common ancestor, but a shared one, nonetheless. Now, research findings published in this week's issue of Nature suggest that ancestor looked an awful lot like a shark.

The findings are based on the skull-analyses (based on molds like the ones featured up top) of a Paleozoic-era fish named Acanthodes bronni. This foot-long, plankton-munching ocean-dweller lived closed to 300 million years ago, but is one of the most well-preserved species belonging to a class of fishes known as Acanthodii, the earliest known creatures on Earth with jaws as well as backbones. When researchers led by biologist Michael Coates compared the reconstructed cranial and jaw anatomy of A. bronni to scans of skulls from early sharks and bony fishes, they came to a surprising realization. LiveScience's Stephanie Pappas explains:

This closer-than-ever look revealed ridges and grooves never before examined. The researchers took 138 characteristics of the skulls and compared them with both the skulls of the chondrichthyes, the group made up of sharks and rays, and the osteichthyes, or bony fish such as today's sardines and mackerel. They found that on the whole, acanthodian heads fell in with the sharks.

"For the first time, we could look inside the head of Acanthodes, and describe it within this whole new context," explained Coates. "The more we looked at it, the more similarities we found with sharks."